Added note on ButtonGroup values handling

ButtonGroup.php

@@ -33,8 +33,12 @@ use yii\helpers\Html;
  *     ]
  * ]);
  * ```
+ *
+ * Pressing on the button should be handled via JavaScript. See the following for details:
+ *
  * @see http://getbootstrap.com/javascript/#buttons
  * @see http://getbootstrap.com/components/#btn-groups
+ * 
  * @author Antonio Ramirez <amigo.cobos@gmail.com>
  * @since 2.0
  */